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Ground-level
ozone concentrations. Ozone is one of several key air pollutants
of concern to human health. Elevated concentrations of ozone can also
harm plants.(Figs. 1, 2)
Particulate
matter concentrations. Fine particulate matter in the air can harm
human health and impair visibility. (Figs. 3,
4)
Visibility
impairment, as a direct measurement of loss of a feature people
value highly. (Figs. 5, 6)
Ozone concentrations
have declined since 1988, but have been relatively stable over most
of the decade. Average concentrations increased more than 15 percent
in five cities or counties, and decreased more than 15 percent in 38
cities or counties. Most had no or only insignificant change. About
half of all individual monitoring sites are in violation of the recently
promulgated federal standard.
Particulate matter
concentrations have dropped considerably and steadily over the past
decade. Average concentrations declined in about 70 percent of all cities
and counties, and the remainder stayed stable or had no significant
change; none increased. More than 95 percent of the individual monitoring
sites are below the federal standard for large particles. Data are scanty
on the fine particle standard recently promulgated by EPA. [Graphics
on following page]
Visibility improved
significantly, especially in the eastern and southern United States,
The most dramatic improvements were in areas that previously had the
most impaired visibility. [Graphics on following pages]
| Ground
Level Ozone Concentrations and Change, 1988-1997 (1) Technical
Note |
The map and
graph display a standard US EPA measure of ozone on the worst
days of the year: the fourth highest daily 8-hour average concentration.
The ozone
map displays the average concentrations for a city or non-urban
county, and provides both the concentration in 1997 and whether
the concentration has been rising or falling over the last decade.
The graph summarizes trends for all monitors (i.e. not city or
county averages) over the past decade, showing the median sites,
poorer than average sites (worst 5 percent and 25 percent of all
monitors), and better than average sites (best 25 percent.)
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| SOURCE:
Environmental Protection Agency |
How to
read this map:
Each
symbol displays the average air quality in a city or county. The
color of the symbol shows the air quality in 1997. The shape of
the symbol shows whether air quality over the last decade has: improved
by more than 15 percent, declined by more than 15 percent, stayed
about the same, or had no statistically significant trend. |
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| National
Trends in Ozone Concentration (2) Technical
Note |
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| SOURCE:
Environmental Protection Agency` |
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STATUS
OF DATA & OTHER NOTES
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Ozone and
particulate matter maps present average concentrations for a city
or county, obtained by averaging values from all monitors within
the city or county. National trends graphs for ozone and particulate
matter present the values from individual monitors, not city or
county averages. Violations of the National Ambient Air Quality
Standards are determined on a monitor-specific basis, not on the
basis of city/county averages.
In 1997,
EPA issued an additional particulate matter standard for finer
particles, i.e., those 2.5 micrometers or smaller. Data on current
concentrations and trends of this size class are insufficient
to display.
The collection
and reporting procedure for visibility at airports was changed
in 1996. Visibility data are now stored with an upper bound of
10 miles, even though the new instrumentation is capable of resolving
visibility in the 20 to 30 mile range. The maps displayed here
will not be available for future periods unless the archiving
methods for visibility data are changed.
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